remove the "defaulted unit" type bit during writeback

The defaulted unit bit is only relevant for the surrounding inference
context, and can cause trouble, including spurious lints and ICEs,
outside of it.

Fixes rust-lang#43853.

src/librustc/infer/resolve.rs

@@ -111,8 +111,10 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for FullTypeResolver<'a, 'gcx, 'tcx>
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
-        if !t.needs_infer() {
+        if !t.needs_infer() && !ty::keep_local(&t) {
             t // micro-optimize -- if there is nothing in this type that this fold affects...
+                // ^ we need to have the `keep_local` check to un-default
+                // defaulted tuples.
         } else {
             let t = self.infcx.shallow_resolve(t);
             match t.sty {
@@ -131,6 +133,12 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for FullTypeResolver<'a, 'gcx, 'tcx>
                 ty::TyInfer(_) => {
                     bug!("Unexpected type in full type resolver: {:?}", t);
                 }
+                ty::TyTuple(tys, true) => {
+                    // Un-default defaulted tuples - we are going to a
+                    // different infcx, and the default will just cause
+                    // pollution.
+                    self.tcx().intern_tup(tys, false)
+                }
                 _ => {
                     t.super_fold_with(self)
                 }

src/librustc/ty/context.rs

@@ -1195,7 +1195,7 @@ macro_rules! direct_interners {
     }
 }
 
-fn keep_local<'tcx, T: ty::TypeFoldable<'tcx>>(x: &T) -> bool {
+pub fn keep_local<'tcx, T: ty::TypeFoldable<'tcx>>(x: &T) -> bool {
     x.has_type_flags(ty::TypeFlags::KEEP_IN_LOCAL_TCX)
 }
 

src/librustc/ty/flags.rs

@@ -151,7 +151,10 @@ impl FlagComputation {
                 self.add_ty(m.ty);
             }
 
-            &ty::TyTuple(ref ts, _) => {
+            &ty::TyTuple(ref ts, is_default) => {
+                if is_default {
+                    self.add_flags(TypeFlags::KEEP_IN_LOCAL_TCX);
+                }
                 self.add_tys(&ts[..]);
             }
 

src/librustc/ty/mod.rs

@@ -76,7 +76,7 @@ pub use self::sty::TypeVariants::*;
 pub use self::binding::BindingMode;
 pub use self::binding::BindingMode::*;
 
-pub use self::context::{TyCtxt, GlobalArenas, tls};
+pub use self::context::{TyCtxt, GlobalArenas, tls, keep_local};
 pub use self::context::{Lift, TypeckTables};
 
 pub use self::instance::{Instance, InstanceDef};

src/test/run-pass/issue-43853.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::panic;
+
+fn test() {
+    wait(|| panic!());
+}
+
+fn wait<T, F: FnOnce() -> T>(f: F) -> F::Output {
+    From::from(f())
+}
+
+fn main() {
+    let result = panic::catch_unwind(move || test());
+    assert!(result.is_err());
+}